Several thermo-mechanical processing and manufacturing techniques have been developed to optimize the microstructure of NiTi alloys and provide superior mechanical properties to the instruments [3, 17]. Thermal treatment may significantly increase the fatigue resistance of NiTi files [18]; the heat treatments change the crystalline disposition of the nickel and titanium atoms, which undergo a main shift from the cubic (austenitic) to tetragonal molecular arrangement (martensitic) phase, inducing different mechanical properties [17, 19–22]. In the present study, five novel heat-treated NiTi files were compared with respect to their cyclic fatigue resistance and separation patterns in single and double curved canals. The main finding of our study was that CM files displayed significantly higher cyclic fatigue resistance in an S-shaped artificial canal, denying the null hypothesis is that there is no difference between cyclic fatigue resistance between all files tested.
Teeth with S-shaped (double curvatures) root canals can be challenging, since the instruments can be vulnerable to deviations in anatomy, loss of working length, and potential for file separation. Fatigue has been implicated to be the main reason for the fracture of clinically used endodontic files [23]. Other studies have tested cyclic fatigue in various instruments in vitro in double curvatures [24], however in the present study, two artificial canals (single and double curvatures) were specially manufactured and their cross-sectional measurements were designed to be suitable for all systems to minimize any factors that may affect the results. Extracted teeth were not used, since imitating clinical conditions and ensuring standardization is difficult with extracted teeth [25]. Nevertheless, the artificial canal has greater volume in comparison to the instruments which allow then to rotate with no low contact area, which is considered a limitation in all studies of this nature but do provide important initial information in file testing.
Fatigue resistance was compared using the TTF and NCF values. In both single- and double-curvature canals, TTF was the longest for CM, followed by EDM, VTB, PTN, and finally OC. CM’s TTF was almost double the TTF for EDM and 7 times higher than the lowest values (PTN and OC). CM also showed the highest NCF in the single-curvature assessments and double-curvature assessments. Controlled memory (CM) wire is produced by a special thermo-mechanical process involving the alloy that controls the memory of the material, making the files extremely flexible but without the shape memory, in contrast to conventional NiTi instruments. This imparts high fatigue resistance to the CM [14, 26], allowing it to easily bend during use and recover its original shape when heated above the transformation temperature [27]. Our present results agree with many studies that have demonstrated that CM wire instruments have greater cyclic fatigue resistance than conventional NiTi instruments [14, 26, 28, 29]. However, the standard deviation for the TTF in the CM group was very high. CM showed similar numbers in the double-curve canal, while the numbers differed in the single-curve canal. This could be due to a manufacturer’s defect in one batch of the files, or it may indicate that despite the high resistance to fracture in this type of file, its reliability is questionable. It is important to note that all tests were performed by the same operator, and all other standard deviations were low, indicating that the possibility of experimenter error is low.
EDM are also produced with CM wire; however, they are produced with a different fabrication process that uses an electro-discharge machining (EDM) process [12]. In the EDM process, melting and evaporation of the top layer creates a rough, crater-like surface finishing [12, 30]. EDM files are the first endodontic instruments manufactured with this process. The surfaces created through the EDM process could improve the cutting efficiency of tools, as compared to conventional NiTi endodontic files [31]. However, some studies have reported that this surface finish has an effect on the crack initiation process [32] and structural irregularities of the instrument surface may compromise its integrity during clinical use, making the files more susceptible to fracture [7], while others have shown that the cyclic fatigue associated with EDM was significantly greater than that for the conventionally manufactured CM files [20]. In the present study, EDM files showed better resistance to cyclic fatigue than the other files, which is in agreement with the findings of several studies [12, 33, 34], but its resistance was still lower than that of the CM files, which is inconsistent with other findings [35]. On the other hand, the EDM file showed a significantly longer fractured fragment length and separated at a larger diameter than all other files in both single and double canal curvatures. This supports the theory of compromised integrity due to instrument irregularities. Both files showed a reduced tendency to extrude out of the canal once fractured, which might prove to be an important consideration in cases of roots with open apexes or enlarged apexes due to resorption or presence of a periapical lesion.
A smaller taper and metal mass of the instrument are known to induce better cyclic fatigue resistance [12, 36–38]. Therefore, the NiTi files that were chosen for this study had comparable taper and size and were used for a single file canal preparation. These systems use a reciprocating motion and claim to shape root canals with a single file [39], which amounts to shorter working time; shorter learning curve, and a reduction in the number of instruments needed [2]. However, single-shaping files are exposed to high levels of stress, which might lead to cyclic and torsional fatigue [40]. Therefore, files used in single-file canal preparation systems should have a high resistance to fracture. In addition, cyclic flexural fatigue resistance decreases with extended use, which might be the case if these files are used for single use [41].
A lubricant was used to reduce friction during the test, as its importance was established in a pilot study and other studies [42]. Although current studies have shown that temperature was found to significantly increase the cyclic fatigue of nickel-titanium rotary files, and reduce the NCF, suggesting that cyclic fatigue studies should be conducted at body temperature (Dosanjh et al. 2017; Klymus et al. 2019), our studies were conducted at room temperature for initial testing.
In order to study the failure mechanism of the test files, the cross-sections of the broken files were viewed under the SEM. Although, 3D replicas could be used to compare cyclic fatigue among files with duplicated reconstructed anatomy, the SEM provides important information regarding fatigue failure within the instruments, which is the reason behind the current real-life replica testing apparatus. EDM showed the highest difference between the diameter of fracture in single and double curvature, the highest being in the single curvature, while the smallest difference was in the VTB, and CM files. The analysis revealed fatigue failure of the test files with crack initiation at one or more points. All files exhibited two distinct regions, one with the fatigue striations (region a) and the other with the dimpled surface (region b) as shown in Fig. (3,4). The fatigue striations are the characteristic feature of fatigue crack growth whereas the dimpled surface indicate ductile facture. The crack originates at the edge due to the presence of high stresses as shown in Fig. (3a, 4a) and it propagates until the fatigue striations. Micro-voids coalescence takes place by the nucleation of micro-voids, which results in weakening of the material, after that ductile fracture occurs until the failure that is evident from the dimpled surface as seen in Fig. (3b, 4b).